1. Technical Field of the Invention
The present invention relates generally to tire pressure monitoring systems. More particularly, the invention relates to a tire pressure monitor, which can accurately detect the state of motion of a vehicle and transmit a pressure signal representative of the inflation pressure of a tire on the vehicle according to the detected state of motion, and a tire pressure monitoring system including a plurality of such tire pressure monitors.
2. Description of the Related Art
Conventional direct-type tire pressure monitoring systems generally include at least one tire pressure monitor and a central monitor.
The tire pressure monitor is directly installed to a wheel of a vehicle and includes a pressure sensor working to sense inflation pressure of a tire fitted on the wheel. The tire pressure monitor is configured to transmit a pressure signal indicative of the inflation pressure of the tire sensed by the pressure sensor.
The central monitor is installed to the body of the vehicle and includes at least one antenna. The central monitor is configured to receive, via the antenna, the pressure signal transmitted from the tire pressure monitor and determine the inflation pressure of the tire based on the received pressure signal.
Japanese Patent First Publication No. 2002-264618 discloses a tire pressure monitor (or tire pressure sensor) for use in a direct-type tire pressure monitoring system. The tire pressure monitor includes an acceleration sensor for detecting the state of motion of the vehicle and is configured to transmit the pressure signal according to the detected state of motion.
More specifically, the acceleration sensor includes a fixed electrode and a movable electrode opposed to the fixed electrode. The acceleration sensor is configured to sense a centrifugal acceleration through detecting a change in the electric capacitance between the fixed and movable electrodes; the change is caused by a displacement of the movable contact due to the centrifugal acceleration which increases with the running speed of the vehicle. The tire pressure monitor is configured to determine the state of motion of the vehicle based on the centrifugal acceleration sensed by the acceleration sensor and transmit the pressure signal according to the determined state of motion. For example, the tire pressure monitor may be configured to transmit the pressure signal only when the vehicle is in running state, thereby reducing the electric power consumption of the tire pressure monitor.
Moreover, the determination as to whether the vehicle is in running or stop state can be made through determining whether the centrifugal acceleration sensed by the acceleration sensor exceeds a predetermined threshold. However, the sensing accuracy of the acceleration sensor is usually influenced by the temperature characteristic and aged deterioration of the acceleration sensor; thus, in some cases, it is difficult to accurately determine whether the vehicle is in running or stop state when the vehicle is running at a low speed.
More specifically, since the tire pressure monitor is mounted on the wheel, it rotates with the wheel and thus receives a centrifugal force during the rotation. Referring to FIG. 9, the centrifugal force is an inertia force which has a radially outward direction and a magnitude F that can be determined by the following equation:F=m×r×ω2  (Equation 1)where, m is the mass, r is the rotating-radius, and ω is the angular rate of the tire pressure monitor.
Accordingly, the acceleration sensor of the tire pressure monitor senses the centrifugal acceleration G which is equal to r×ω2. Further, since the angular rate ω increases with the running speed V of the vehicle, the centrifugal acceleration G has the following relationship with the running speed V:G=A×R×V2  (Equation 2)where, R is the radius of the wheel, and A is a constant which depends on the type of the wheel and the mounting position of the tire pressure monitor on the wheel.
As shown in FIG. 10, the centrifugal acceleration G exponentially increases with respect to the running speed V of the vehicle. Accordingly, it is possible to determine the running state of the vehicle based on the centrifugal acceleration G sensed by the acceleration sensor.
However, when the acceleration sensor is configured with a low-cost acceleration sensor which has an accuracy tolerance of ±α%, there will be upper and lower allowable limits of error as shown in FIG. 11.
Further, considering the sensing error caused by the aged deterioration of the acceleration sensor, which is in the range of ±β(G), the sensed centrifugal acceleration may deviate from the actual centrifugal acceleration within the range between upper and lower limits of error as shown in FIG. 12.
Consequently, due to the deviation of the sensed centrifugal acceleration from the actual one, it is difficult to suitably predetermine the threshold for determination of the state of motion of the vehicle.
For example, to detect running state of the vehicle even when the vehicle is running at a low speed, it is desirable to set the threshold as low as possible. However, when the threshold is set so as to be lower than Gr shown in FIG. 13, the vehicle may be determined as being in running state, though it is actually in stop state, due to the sensing error of the acceleration sensor. Thus, the pressure signal may be transmitted even when the vehicle is in stop state, thereby increasing the power consumption of the tire pressure monitor.
On the contrary, when the threshold is set so as to be higher than Gr shown in FIG. 13, the vehicle may not be determined as being in running state unless it is running at a considerably high speed. Thus, when the sensed centrifugal acceleration deviates from the actual one toward the lower limit of error, the vehicle may be determined as being in stop state though it is actually running at a low speed. FIG. 14 illustrates an example of such incorrect determination, wherein the hatched zones represent the time periods for which the vehicle is determined as being in stop state though it is actually in running state.